Apparatus and method for handling easily polymerizable substance, apparatus for extracting liquid from apparatus under reduced pressure, and process for producing easily polymerizable substance

ABSTRACT

In an apparatus capable of distilling and refining an easily polymerizable substance, piping is performed so that when a strainer in action is switched to a backup strainer, the strainer can be switched after removing the air in the backup strainer to replace the air in the backup strainer to a discharge liquid of a pump. According to the apparatus, switching the strainers installed on the upstream side of the pump for extracting a liquid of a vacuum distillation column which distills and refines the easily polymerizable substance can be performed with no trouble in the operation of the distillation column. When the apparatus comprises a means measuring a differential pressure between the upstream side and the downstream side of the strainer in action, the two or more strainers installed in parallel can be switched at proper times and the easily polymerizable substance can be produced efficiently.

TECHNICAL FIELD

[0001] The present invention relates to an apparatus for extracting a liquid, particularly a liquid potentially containing solids, from an apparatus under reduced pressure, and to a process for producing an easily polymerizable substance which comprises switching two or more strainers parallelly installed in a liquid extraction pipe at proper times, the liquid extraction pipe is connected to a pump for transferring an outlet liquid from a treating column for handling the easily polymerizable substance.

BACKGROUND ART

[0002] When distilling and refining easily polymerizable substances, for example, acrylic acid and methacrylic acid or their esters, distillation is performed with addition of polymerization inhibitors and under reduced pressure in order to prevent polymerization from occurring. However, it is impossible to completely prevent the polymerization and it is inevitable that some polymerization occur. In particular, for example, a drastic change in conditions under which the distillation apparatus is operated for any reason may result in an abrupt progress in the polymerization. Therefore, some polymerisate may exist in distillation residue drawn out from the bottom of the distillation apparatus and polymerisate may also be contained in condensate in a reflux tank of the top of the apparatus.

[0003] Extracting the distillation residues of the bottom of the column to the outside, extracting the condensate in the reflux tank of the top of the column to the outside or refluxing the condensate to a distillation column needs aid of a pump. However, direct supply of a liquid that contains solids such as a polymerisate may cause the solids to be attached to a casing part of the pump or come into a mechanical seal part of the pump. Once such a phenomenon occurs, the flow is stagnated at that portion, so that the polymerization of the polymerizable substance in the distillation residue proceeds, thus causing a serious trouble in the operation of the pump, and ultimately it becomes inevitable to stop the operation of the pump.

[0004] Further, it is possible that the solids that passed the pump may cause clogging of a flow meter and control valve provided on the discharge side of the pump. Therefore, a strainer is provided usually upstream side of the pump in order to avoid feeding of solids to the pump. To allow for continued operation of the distillation column even when the strainer is clogged with solids, two or more strainers are provided parallelly. In the case where one strainer which is in operation is clogged, a backup strainer provided parallelly can be immediately brought in operation by switching the valves.

[0005] However, in general, switching a strainer of a pump for extracting a liquid from a vacuum distillation apparatus to a backup strainer tends to cause an abnormal operation of the pump. Also, in the case of distillation and refining of a polymerizable substance, switching strainers of a pump for extracting a condensate from a reflux tank may result in production of a polymerisate in the condensate in the reflux tank. Also in the case of switching the strainers of a pump for extracting distillation residue from the bottom of a column, production of polymerisate in the column may increase.

[0006] The inventors of the present invention have studied the cause therefor and as a result, they have found that such is attributable to the air contained in the backup strainer. That is, the strainer clogged with solids becomes a backup strainer to open and clean. These operations are performed in the air and hence atmospheric air is present in the backup strainer. Accordingly, operation of the backup strainer will result in back flow of the inside air to the upstream side to flow into the apparatus under reduced pressure or flow into the pump located downstream or further into an apparatus to which the discharge liquid of the pump flows in. As a result, the operation of the pump is disturbed and the operation of the pump must be stopped as the case may be.

[0007] Further, the disturbed operation of the pump or flown in air will disturb the flow of steam and liquid within the distillation apparatus, with the result that polymerization of the polymerizable substance is promoted. That is, although a polymerization inhibitor is fed to a condenser of the top of the column to prevent the condensate from polymerizing when distilling and refining a polymerizable substance, it is conceivable that disturbed flow of steam in the condenser will produce a condensate in which the concentration of the polymerization inhibitor is locally reduced, which triggers off polymerization.

[0008] Further, back flow into the distillation column of the air in the backup strainer of a pump for extracting the distillation residues from the bottom of a column will disturb the flow of the steam and liquid in the column, causing local stagnation of the liquid or local concentration reduction of the polymerization inhibitor to promote the polymerization.

[0009] In a process where acrylic acid, methacrylic acid, and esters thereof, which are typical easily polymerizable substances, are produced by catalytic gas phase oxidation, it is important to stabilize operational conditions of the respective unit operation apparatuses used in unit operations such as separation or purification of acrylic acid, methacrylic acid and esters thereof, from the viewpoint of stable producing of them which are the easily polymerizable substances, in order to suppress formation of polymerisate that will frequently occur during the operation.

[0010] Since they are easily polymerizable substances, polymerisate is generated in the inside of the plant in spite of the use of a polymerization inhibitor or a polymerization retardant. For example, polymerisate is produced in each of the inside of the distillation column and condenser that constitute the process and the polymerisate is accumulated in the inside of the distillation column or a part of or the whole polymerisate falls to the bottom of the column, or clogs the condenser, or falls to the reflux tank or the pump connected to the reflux tank through a line.

[0011] To remove the polymerisate, for example, a strainer is installed on the feed side of a pump for transferring a column bottom liquid that transfers the column bottom liquid to a downstream appliance is provided for the purposes of protecting the pump, preventing polymerization solids from being fed to the appliance downstream of the pump, and the like. Further, since continuous operation is intended, two or more strainers parallelly installed (one strainer being in action and other strainer(s) being in a standby state) are provided.

[0012] In the case where an easily polymerizable substance is produced, although the polymerisate generated can be collected by the strainer and removed out of the process thereby, the times of switching to the strainer provided as a backup cannot be judged since the state of the polymerisate being collected during the operation of the strainer cannot be grasped. That is, an increased amount of collection will cause clogging of the strainer with the collected polymerizate, thus generating cavitation in the pump connected to the strainer. This causes abnormality of pump operation, which in turn leads to disturbed operational conditions (for example, variation in pressure, variation in flow rate, and variation in temperature) in appliances installed on upstream and downstream side of the pump, thereby generating polymerisate in the appliances to stop the operation of the appliances or the entire plant.

[0013] For this reason, the strainers are frequently switched to cope with the aforementioned problems; however, this brings a labor load of an operator who operates and in the case where the substance to be handled is a dangerous substance, toxic substances, or the like, utmost care must be taken and the operator is put under a great psychological burden.

[0014] Easily polymerizable substances, particularly acrylic acid, methacrylic acid, and esters thereof have this tendency to such a significant extent. And in an industrial production plant, a countermeasure is demanded to reduce the burden on the operator and avoid the situation where the operation has to be stopped.

DISCLOSURE OF THE INVENTION

[0015] An object of the present invention is to provide an apparatus for extracting a liquid and an apparatus and method for handling an easily polymerizable substance, in which troubles of operation caused by the air contained in such a backup strainer can be avoided.

[0016] Further, another object of the present invention is switching two or more strainers at proper times, the strainers which are parallelly installed in a liquid extraction pipe for transferring the outlet liquid of a treating column from the treating column for handling the easily polymerizable substance to the pump for transferring the outlet liquid, in a process for producing an easily polymerizable substance and an apparatus and method for handling an easily polymerizable substance.

[0017] The apparatus and method for handling an easily polymerizable substance according to the present invention is as follows.

[0018] (1) An apparatus for handling an easily polymerizable substance, comprising: a decompressor for handling the easily polymerizable substance under reduced pressure; a liquid extraction pipe having an upstream end that opens in the decompressor; a pump provided in the middle of the liquid extraction pipe; a plurality of strainers connected in parallel one another to the liquid extraction pipe on the upstream side of the pump; and valves provided on the upstream side and downstream side of each strainer, wherein the apparatus comprises at least one of constructions (A) and (B) below:

[0019] (A) an exhaust means for exhausting air in a part sandwiched between the valves provided on the upstream side and downstream side of each strainer, and a branch pipe that connects the part sandwiched between the valves provided on the upstream side and downstream side of each strainer and the liquid extraction pipe on the downstream side of the pump; and

[0020] (B) a means for measuring a differential pressure between a pressure in the liquid extraction pipe on the upstream side of each strainer and a pressure in the liquid extraction pipe on the downstream side of each strainer.

[0021] (2) The apparatus according to (1), wherein the exhaust means is configured such that when a liquid is introduced from the branch pipe to the part sandwiched between the valves provided on the upstream side and downstream side of each strainer, the liquid expels the air in the part.

[0022] (3) The apparatus according to (1) or (2), wherein the decompressor for handling the easily polymerizable substance under reduced pressure is an apparatus for distilling the easily polymerizable polymer selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof under reduced pressure.

[0023] (4) A method for handling an easily polymerizable substance using the apparatus according to any one of (1) to (3), comprising: using the plurality of strainers one by one; determining the time for switching the strainers to measure a difference between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used; and switching the strainer used to another strainer out of the plurality of strainers.

[0024] (5) The method according to (4), further comprising: discharging air in a part sandwiched between valves provided on the upstream side and downstream side of the another strainer; feeding a discharge liquid of the pump from the branch pipe to the part; and switching the strainer used to another strainer after replacing the air in the part with the discharge liquid.

[0025] The apparatus for extracting a liquid from an apparatus under reduced pressure according to the present invention is as follows.

[0026] (6) An apparatus for extracting a liquid from an apparatus under reduced pressure, comprising: a liquid extraction pipe for transferring a liquid under reduced pressure contained in an apparatus under reduced pressure, having an upstream end that opens in the apparatus under reduced pressure, and having a pump in the middle of the pipe; a plurality of strainers connected in parallel one another to the liquid extraction pipe on the upstream side of the pump; valves provided on the upstream side and downstream side of each strainer; an exhaust means for exhausting air in a part sandwiched between the valves; and a branch pipe that connects a part sandwiched between the valves provided on the upstream side and downstream side of each strainer and the liquid extraction pipe on the downstream of the pump.

[0027] (7) The apparatus according to (6), wherein the exhaust means is configured such that when a liquid is introduced from the branch pipe to the part sandwiched between the valves provided on the upstream side and downstream side of each strainer, the liquid expels the air in the part.

[0028] (8) The apparatus according to (6) or (7), wherein the apparatus under reduced pressure is a vacuum distillation apparatus for an easily polymerizable substance selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof.

[0029] In the apparatus for extracting a liquid from an apparatus under reduced pressure according to the present invention, switching to a backup strainer is performed as follows. First, the discharged liquid from the pump is introduced to the backup strainer through a branch pipe by switching valves, the air in the backup strainer is replaced by the discharged liquid, and then the backup strainer is brought in action. Therefore, the disturbed operation due to the air in the backup strainer can be avoided.

[0030] Further, the inventors of the present invention have made diligent study on the aforementioned problem and as a result they have found that proper times for switching strainers can be determined by measuring a differential pressure between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used in order to grasp the condition of the strainer. Thus, the present invention is accomplished.

[0031] That is, the process for producing an easily polymerizable substance according to the present invention is as follows.

[0032] (9) A process for producing an easily polymerizable substance using a treating apparatus having one or two or more treating columns for treating the easily polymerizable substance, a pump provided in each treating column for transferring an outlet liquid of the treating column, and two or more strainers installed parallelly in a liquid extraction pipe connecting the treating column and the pump, wherein the process comprises:

[0033] using the two or more strainers one by one in the each treating column; determining the time of switching the strainers to measure a difference between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used; and switching the strainer used to another strainer out of the two or more strainers.

[0034] (10) The process according to (9), wherein the differential pressure of the strainer is continuously measured.

[0035] (11) The process according to (9), wherein extract nozzles for connecting a detection end for measuring pressure are installed to the top of the liquid extraction pipe of the strainer used.

[0036] (12) The process according to any one of (9) to (11), in which the easily polymerizable substance is selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof.

[0037] (13) The process according to any one of (9) to (12), wherein the treating apparatus comprises the treating column selected from any one of a distillation column, an evaporation column, a reflux tank, an extraction column, an absorption column, a high boiling point decomposition reactor, an esterification reactor, a heat exchanger or combinations thereof.

[0038] <An Apparatus of the Present Invention for Extracting a Liquid from an Apparatus Under Reduced Pressure>

[0039] The present invention will be described in more detail with reference to the attached drawings. FIG. 1 shows an example of the apparatus for extracting a liquid according to the present invention.

[0040] Reference numerals 1 and 2 stand for strainers. The strainers may be those employed in general chemical apparatus. For example, Y type strainers, bucket type strainers, and the like are used.

[0041] Reference numeral 3 indicates a pump. Similarly to the strainer, a backup pump may be provided parallelly as the case may be. Reference numeral 4 indicates an upstream side liquid extracting pipe, 5 indicates a downstream side liquid extracting pipe, and 6 is a branch pipe.

[0042] The upstream side liquid extracting pipe 4 is a pipe for transferring a liquid under reduced pressure contained in an apparatus under reduced pressure and has an upstream end opening in the apparatus under reduced pressure though not shown. In the present invention, “upstream end opening in the apparatus under reduced pressure” means that the upstream end of the liquid extracting pipe opens in a chamber, column and the like in which the liquid under reduced pressure is contained. The apparatus under reduced pressure includes, for example, a vacuum distillation apparatus, specifically the apparatus shown in FIG. 2 or FIG. 4. The substances handled in such an apparatus include, for example, easily polymerizable substances selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof.

[0043] A valve a and a valve b are provided upstream and downstream of the strainer 1 respectively, and the strainer 1 is also provided with an exhaust valve h. Also, a valve c and a valve d are provided upstream and downstream of the strainer 2 respectively, and the strainer 2 is also provided with an exhaust valve i.

[0044] The exhaust valves h and i are used for expelling and discharging the air in the part sandwiched between the valves a and b, or c and d by the discharged liquid from the pump 3 fed through the branch pipe 6; therefore, the exhaust valves h and i are provided at the highest position between the part sandwiched between the valves a and b, or c and d.

[0045] Note that in FIG. 1, the branch pipe 6 opens between the strainer 1 and the valve a. However, it may open between the strainer 1 and the valve b. The same is true for the strainer 2; the branch pipe 6 may be opened on the downstream side of the strainer 2.

[0046] The action of the liquid extraction apparatus is described as follows. When the strainer 1 is in action, all the valves are closed except for the valves a and b that are open. Upon switching from the strainer 1 to the strainer 2, the valves i and g are opened and then the valve e is slowly opened to allow the discharged liquid from the pump to flow into the strainer 2 through the branch pipe 6. Inclusive of the strainer 2, the air in the part sandwiched between the valves c and d is discharged form the exhaust valve i by being pushed by the discharge liquid.

[0047] When the part sandwiched between the valves c and d is completely replaced by the discharge liquid, the valves e, g and i are closed, and the valves c and d are opened to bring the strainer 2 in action. Then, the valves a and b are closed and the strainer 1 is separated from the line. Subsequently, the valves j and h are opened to draw out the liquid in the inside of the strainer 1. Thereafter, the strainer 1 is opened and the inside thereof is cleared to obtain a backup strainer. It is preferable that the liquid in the inside of the branch pipe 6 is drawn out by opening a valve k.

[0048] <A Process for Producing an Easily Polymerizable Substance According to the Present Invention>

[0049] Hereinafter, the process for producing an easily polymerizable substance according to the present invention will be described in detail.

[0050] The present invention is a process for producing an easily polymerizable substance using a treating apparatus including one or two or more treating columns for handling an easily polymerizable substance, a pump provided to each treating column for transferring an outlet liquid from the treating column, and two or more strainers parallelly installed in a liquid extraction pipe from the treating column to the pump, wherein: in each treating column, one each of the two or more strainers is used, a time for switching the strainers is determined by measuring a difference between the pressure upstream side of the strainer of the liquid extraction pipe used on the strainer side and the pressure downstream side of the strainer of the liquid extraction pipe, and the one strainer is switched to another strainer out of the two or more strainers.

[0051] Examples of typical easily polymerizable substances include acrylic acid, methacrylic acid, and esters thereof. Examples of the acrylates include methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, tertiarybutyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and methoxtethyl acrylate. Examples of the methacrylates include methyl methacrylate, butyl methacrylate, isobutyl methacrylate, tertiarybutyl methacrylate, and 2-hydroxyethyl methacrylate.

[0052] In the treating apparatus used in the present invention, the treating column for handling an easily polymerizable substance includes any one of or combinations of a distillation column, an evaporation column, a reflux tank, an extraction column, an absorption column, a high boiling point decomposition reactor, an esterification reactor, and a heat exchanger. In the case of combinations of two or more of them, each of the apparatus that constitute the treating column is provided with a pump for transferring an outlet liquid and two or more strainers parallelly installed in the liquid extraction pipe of the pump from the treating column.

[0053] The pump employed in the present invention is used for transferring the outlet liquid from the treating column to the subsequent appliance and specific examples thereof include a general reciprocating pump, a rotary pump, a volute pump, an axial flow pump, and a mixed flow pump as well as a specialty pump such as a jet pump and the like.

[0054] In addition, the subsequent appliance includes the aforementioned treating columns for handling easily polymerizable substances in the present invention, such as any one of or combinations of a distillation column, an evaporation column, a reflux tank, an extraction column, an absorption column, a high boiling point decomposition reactor, an esterification reactor, and a heat exchanger as well as tanks, waste water disposing appliance, incinerators and the like.

[0055]FIG. 2 shows one example of the appliance that constitutes the treating apparatus for handling the easily polymerizable substances. Specifically, the treating apparatus including a distillation column for producing acrylic acid, methacrylic acid, and esters thereof as a treating column is shown.

[0056] The treating apparatus shown in FIG. 2 includes distillation columns installed in series or parallel, a pump for transferring an outlet liquid from each distillation column, and two or more strainers parallelly installed in the liquid extraction pipe connecting each distillation column and the pump (provided that in FIG. 2, one distillation column and one pump are illustrated).

[0057] In some cases, an evaporation column, an extraction column and the like may be installed as necessary between the distillation columns and each of these is installed with a pump for transferring outlet liquid and two or more strainers parallelly installed in the liquid extraction pipe connecting the distillation column and the pump.

[0058] The liquid fed from an introduction line 7 to a distillation column 8 is distilled. The distilled gas is transferred from the outlet on the top of the column to a condenser 9 where it is cooled. Then a liquid is transferred to a reflux tank 10. A portion of the liquid in the reflux tank 10 is returned to the distillation column 8 via the liquid extraction pipe 11 for transferring the outlet liquid from the reflux tank 10 and pump 12 through a reflux line 13. The remainder is extracted from a column top extraction line 14 and transferred to the subsequent appliance.

[0059] A bent gas is transferred to a bent condenser 16 through a bent line 15 to be cooled and after valuable substances in the gas are recovered, the bent gas is introduced into a vacuum appliance 17 as the case may be. Depending on the operational conditions of the treating apparatus, the vacuum appliance 17 is unnecessary.

[0060] Here, the valuable substance indicates an organic substance handled in the distilling column 8. For example, when manufacturing acrylic acid, the valuable substances include acrylic acid, the solvents used when manufacturing acrylic acid (such as methyl ethyl ketone, methyl isobutyl ketone, toluene, and isopropyl acetate). When manufacturing acrylates, the valuable substances include methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, tertiarybutyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and methoxyethyl acrylate and the like; methanol, ethanol, 2-ethylhexanol, methoxyethanol and the like as the raw materials; and benzene, toluene and the like as the solvent.

[0061] A condensate and non-condensed gas are discharged from the vacuum appliance 17 through an exhaust line 18. Since a small amount of valuable substances are contained in the exhaust line 18, the condensate and the non-condensed gas are recycled to the production process for acrylic acid, methacrylic acid, and esters thereof as the case may be.

[0062] To transfer the column bottom liquid (outlet liquid) of the distillation column 8 to a subsequent appliance, the outlet liquid is extracted through a liquid extraction pipe 19 and a pump 20 and column bottom extraction line 21. A portion of the column bottom liquid is returned to the distillation column 8 via a reboiler 23 through a reboiler circulation line 22.

[0063] Since FIG. 2 is a schematic drawing, a polymerization inhibitor liquid extraction pipe, and an air liquid extraction pipe that are necessary depending on the conditions of the system in distillation of acrylic acid, methacrylic acid, and esters thereof are omitted.

[0064] Referring to FIG. 3, explanation will be made of a monitoring system for conditions of two or more strainers parallelly installed in the liquid extraction pipe to the pump for transferring the outlet liquid of the treating column from the treating column to a subsequent appliance.

[0065] The aforementioned strainers are parallelly installed for the treating columns and the corresponding pumps in the number of two or more for one pump in the liquid extraction pipe. During operation, the strainers are used one by one. The present invention is characterized in that determining a time of switching the strainers to measure a difference between the pressure on the upstream side of the strainer used of the liquid extraction pipe and the pressure on the downstream side of the strainer used of the liquid extraction pipe, and switching the strainer to another strainer.

[0066] The number of strainers installed in one pump is not particularly limited; usually three or less, preferably two. The number of strainers of four or more is not cost effective since the number of strainers that do not participate in the operation is increased. Note that the size of the strainers may be the same or different.

[0067] The liquid is fed from the treating column (in FIG. 2, the distillation column 8, reflux tank 10, etc.) via a liquid extraction pipe 24 to the pump 3. In FIG. 3, two strainers, the strainers 1 and 2 are installed parallelly with respect to the pump 3 and the strainer 1 is used and the strainer 2 is backup with the valves a, b, 1, and o are open and the other valves are closed.

[0068] A difference between the pressure of liquid extraction pipe 24 that is the pressure of the upstream side of the strainer (a pressure detection part 25 of the liquid extraction pipe) and the pressure of the liquid extraction pipe 26 that is the pressure of the downstream side of the strainer (a pressure detection part 27 of the liquid extraction pipe) is observed on a differential pressure gauge 28. The observed information is transmitted to an instrument room installed in the treating apparatus. In the case where entrapment of the polymerisate is in small amounts, it is sufficient that the differential pressure can be observed in situ. In FIG. 2, as a cable for transmitting signals from a detection terminal as observed information, pressure information transmission lines 29 and 30 are installed.

[0069] As the polymerisate is accumulated in the strainer 1, the difference between the pressure of the liquid extraction pipe 24 (the pressure detection part 25 of the liquid extraction pipe) and the pressure of the liquid extraction pipe 26 (the pressure detection part 27 of the liquid extraction pipe) is increased. Judgment of the time of switching when the difference comes to be increased depends on the condition of the system (capability of the pump, the composition of liquid in each line, the diameter of the pipe in each line, etc.) and hence is not limited. Generally, 1.5 to 150 times, preferably 1.5 to 100 times, more preferably 1.5 to 50 times based on the differential pressure in a state where no polymerisate is present (initial stage of the operation of the strainer).

[0070] The strainers used in the treating apparatus in the present invention are not particularly limited for the type; any one of those used in general chemical plants may be used. Specific examples thereof include Y type strainers, bucket type strainers, T type strainers, etc.

[0071] Format and differential pressure gauge for the detection of pressures for measuring the differential pressure of the strainers in the present invention are not particularly limited and any one of those used in general chemical plants may be employed. Specifically, examples of a pressure detector include a diaphragm type pressure gauge. Examples of the differential pressure gauge include a diaphragm displacement type electronic pressure difference transmitter (generally referred to as a capillary type). For example, EDR-7 manufactured by Hitachi Ltd. corresponds to this.

[0072] The position of installing extraction nozzles (31 and 32 in FIG. 2) for connecting a pressure detection end to the liquid extraction pipe for measuring a differential pressure of the strainer in the present invention is preferably in an upper part of the liquid extraction pipe of the strainer used (liquid extraction pipes 24 and 26 in FIG. 2). Although detection is possible even when the nozzles are installed in the same horizontal position as the liquid extraction pipes 24 and 26, or in the lower part, influence of the liquid flow in the liquid extraction pipe are likely, so that installing in the upper part is optimal. The angle of installment of the nozzles with respect to the liquid extraction pipe is preferably 5 to 175°, more preferably 70 to 110°, in the upper part of the liquid extraction pipe. The optimal angle is 90°.

[0073] Generally, extraction nozzles for connecting the pressure detection end and the liquid extraction pipe are installed. The length of the extraction nozzle is within 800 mm, within preferably 500 mm in terms of horizontal piping. A long extraction nozzle may generate polymerisate due to stagnation of the liquid. The extraction nozzle that connects the liquid extraction pipe and the pressure detection end may be installed with a valve which can open and shut. The infimum of distance is not limited since the pressure detector may be installed directly to the liquid extraction pipe depending on the kind of the pressure detector.

[0074] <An Apparatus and a Method for Handling Easily Polymerizable Substance of the Present Invention>

[0075] An apparatus for handling an easily polymerizable substance of the present invention comprises: a decompressor for handling the easily polymerizable substance under reduced pressure; a liquid extraction pipe having an upstream end that opens in the decompressor; a pump provided in the middle of the liquid extraction pipe; a plurality of strainers connected in parallel one another to the liquid extraction pipe on the upstream side of the pump; and valves provided on the upstream side and downstream side of each of the strainers, and comprises at least one of constructions (A) and (B) below:

[0076] (A) an exhaust means for exhausting air in a part sandwiched between the valves provided on the upstream side and downstream side of each strainer, and a branch pipe that connects the part sandwiched between the valves provided on the upstream side and downstream side of each strainer and the liquid extraction pipe on the downstream side of the pump; and

[0077] (B) a means for measuring a differential pressure between a pressure in the liquid extraction pipe on the upstream side of each strainer and a pressure in the liquid extraction pipe on the downstream side of each strainer.

[0078] The apparatus for handling the easily polymerizable substance of the present invention may have only the construction (A) or only the construction (B) out of the aforementioned constructions (A) and (B), or both the constructions (A) and (B).

[0079] In the case where the apparatus has only the construction (A) out of the constructions (A) and (B), the apparatus for handling the easily polymerizable substance of the present invention can avoid the occurrence of disturbance in operation caused by air contained in the backup strainer.

[0080] In the case where the apparatus has only the construction (B) out of the constructions (A) and (B), the apparatus for handling the easily polymerizable substance of the present invention can switch the two or more strainers parallelly installed in the liquid extraction pipe connecting the pump for transferring the outlet liquid of the treating column and the treating column for handling the easily polymerizable substance at proper times.

[0081] In the case where the apparatus has both the constructions (A) and (B) out of the above-mentioned constructions (A) and (B), it becomes possible to avoid the troubles of operation caused by the air contained in the backup strainer, and to switch two or more strainers parallelly installed in the liquid extracting pipe for transferring the outlet liquid of the treating column from the treating column for handling an easily polymerizable substance to the pump for transferring the outlet liquid of the treating column at proper times.

[0082] The aforementioned decompressor maybe constructed by an apparatus for handling an easily polymerizable substance and an apparatus for forming an atmosphere under reduced pressure. As such a decompressor, for example, an apparatus for distilling an easily polymerizable substance selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof under reduced pressure may be mentioned. As the apparatus for handling the easily polymerizable apparatus may be used the aforementioned treating apparatus. As the apparatus for forming an atmosphere under reduced pressure, a well-known vacuum appliance such as a vacuum pump may be used.

[0083] Note that the construction of the aforementioned decompression is not limited to one that has an apparatus for forming atmosphere under reduced pressure, but may be an apparatus for handling an easily polymerizable substance in an atmosphere under reduced pressure. Such apparatus include, for example, the aforementioned treating column such as a heat exchanger, and the like, connected to a vacuum pump.

[0084] The outlet liquid of the aforementioned decompressor is a liquid extracted from the decompressor. Examples of such a liquid include column bottom liquid of the distillation column and condensate of the gas component obtained by distillation and so forth.

[0085] The upstream end of the aforementioned liquid extraction pipe is open in the decompressor. This means that the upstream end of the liquid extraction pipe is open in a chamber or a column formed so as to be under a reduced pressure atmosphere.

[0086] The aforementioned exhaust means may be a means that forcibly discharges the air in the system such as a vacuum appliance, e.g., a vacuum pump. It is preferable from the view point of simplification of the construction of the apparatus to configure such that a liquid introduced from the aforementioned branch pipe to the part sandwiched between the valves installed upstream side and downstream side of each strainer will expel the air in this part.

[0087] For the aforementioned means for measuring the differential pressure, a well-known means that can measure the pressure of a liquid in a pipe may be used similarly to the differential pressure gauge used in the treating apparatus employed in the method for producing of the easily polymerizable substance of the present invention.

[0088] Further, the aforementioned means for measuring differential pressure is preferably one that is installed by using the aforementioned extract nozzles. Also, in the present invention, the aforementioned means for measuring the differential pressure is preferably provided such that the differential pressure measured can be transmitted to an instrument room for the aforementioned decompressor for continuously measuring the differential pressure of the aforementioned strainers.

[0089] The apparatus for handling an easily polymerizable substance of the present invention may be specifically configured by applying the apparatus shown in FIG. 1 to the apparatus shown in FIG. 2 or FIG. 4, or applying the apparatus shown in FIG. 3 to the apparatus shown in FIG. 2 or FIG. 4.

[0090] A method for handling an easily polymerizable substance of the present invention is a method for handling an easily polymerizable substance using the apparatus for handling an easily polymerizable substance of the present invention, comprises: using the plurality of strainers one by one; determining times for switching the strainers to measure a difference between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used; and switching the strainer used to another strainer out of the plurality of strainers.

[0091] According to this method, the two or more strainers parallelly installed in the liquid extraction pipe connecting the pump for transferring the outlet liquid of the treating column and the treating column for handling the easily polymerizable substance can be switched at proper times.

[0092] Further, when the method for handling an easily polymerizable substance of the present invention further includes: discharging air in a part sandwiched between valves provided on the upstream side and downstream side of the another strainer; feeding a discharge liquid of the pump from the branch pipe to the part, and switching the strainer used to another strainer after replacing the air in the part with the discharge liquid, it also becomes possible to avoid the troubles of operation caused by the air contained in the backup strainer.

[0093] Note that, in the invention, “handling” an easily polymerizable substance means various operations relative to liquid containing an easily polymerizable substance such as production of an easily polymerizable substance, separation of an easily polymerizable substance, washing of an easily polymerizable substance, purification of an easily polymerizable substance, and transportation of an easily polymerizable substance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0094]FIG. 1 is a diagram showing one example of an apparatus for extracting a liquid according to the present invention.

[0095]FIG. 2 is a diagram showing one embodiment of a treating apparatus used in a process for producing an easily polymerizable substance of the present invention.

[0096]FIG. 3 is a schematic diagram showing a pump and its circumference of a treating apparatus used in the present invention.

[0097]FIG. 4 shows one example of a vacuum distillation column to which an apparatus for extracting a liquid according to the present invention is applied.

BEST MODE FOR CARRYING OUT THE INVENTION

[0098] Hereinafter, the present invention will be described in more detail by examples and comparative examples. However, the present invention is not limited to the following examples unless going beyond the gist of the present invention.

EXAMPLE 1

[0099] Explanation will be made of the case where the liquid extraction apparatus according to the present invention was applied to the vacuum distillation apparatus shown in FIG. 4 that purifies acrylic acid to form high purity acrylic acid.

[0100] In FIG. 4, reference numeral 8 stands for a distillation column, 23 stands for a reboiler, 33 stands for a reboiler feed pump, 10 stands for a reflux tank, 12 stands for a pump for extracting a condensate, 20 stands for a pump for extracting a column bottom liquid, and 9 and 16 stand for condensers. At the top and bottom of the column are attached the liquid extraction apparatus shown in FIG. 1 each having a pump 12 and a pump 20. Both the strainers are basket type strainers. Note that in FIG. 4, a polymerization inhibitor feeding apparatus, a vacuum generating appliance, and other ancillary appliance are omitted.

[0101] The aforementioned vacuum distillation apparatus was operated with the distillation column being at a column top pressure of 3 kPa and a column bottom pressure of 11 kPa. During the operation, switching from the strainer 1 to the strainer 2 on the column top and the column bottom of the liquid extraction apparatus was performed in the aforementioned method caused neither change in the operation of the pump nor change in differential pressure between the column bottom and the column top.

[0102] After 3 months, the strainer 2 of the liquid extraction apparatus at the column top and the column bottom was switched to the strainer 1 again in the aforementioned method. Checking the inside of the strainer 2 indicated no polymerisate.

COMPARATIVE EXAMPLE 1

[0103] In contrast, switching from the strainer 1 to the strainer 2 of the column top and the column bottom of the liquid extraction apparatus was performed by the method in which the valves c and d were slowly opened while the other valves remaining closed. Then, both the pumps generated abnormal sounds and the pressure of the distillation column abruptly varied. This is considered because the air in the part sandwiched by the valves c and d including the strainer 2 flew into the distillation column via the pump or flew backward.

[0104] After a while, the abnormal sounds of the pump disappeared and the valves a and b were closed, thus completing the switching of the strainers. Subsequently, continued operation of the distillation column led to a gradual increase in the differential pressure between the column bottom and the column top.

[0105] Then, the strainer 2 of the liquid extraction apparatus at the column top and the column bottom was switched again to the strainer 1. Checking the inside of each strainer 2 indicated polymerisate.

EXAMPLE 2

[0106] By using the treating apparatus with the distillation column 8 as shown in FIG. 2, acrylic acid was purified.

[0107] The distillation column 8 had a column top pressure at 3 kPa and a column bottom pressure of 11 kPa and the differential pressure of the strainers installed in the liquid extraction pipe of the pump for transferring the outlet liquid from the column bottom was continuously measured. Note that the condition of the circumference of the pump was the same as that shown in FIG. 3 except that in this example, two backup strainers were provided. The extract nozzles (31 and 32) for connecting the pressure detection end of the differential pressure gauge to the liquid extraction pipes 24 and 26 were installed at 90° to the liquid extraction pipe and had a length of 150 mm.

[0108] The differential pressure at initial stage of the operation of the strainer was 1.6 kPa. After continuous operation, the differential pressure was increased slowly and reached 19 kPa after 35 days. Then, the strainer was switched and the inside thereof was checked. Then, polymerisate was confirmed.

[0109] The operation of the distillation column was continued, and after 30 days, the differential pressure became 20 kPa so that the strainer was further switched and the inside thereof was checked. Polymerisate was confirmed in the strainer. The operation of the distillation column could be continued further.

[0110] As described above, the time of switching the strainers could be well judged.

COMPARATIVE EXAMPLE 2

[0111] By using the same treating apparatus as that in Example 2 except that the apparatus had no differential pressure gauge as the treating apparatus, the operation was performed. After 42 days, the pump generated cavitation so that the strainer was switched urgently. The inside of the strainer was full of the polymerisate.

[0112] The cavitation resulted in malfunction of the operation of the distillation columns and after 3 days, cavitation of the pump occurred again. The inside of the strainer was full of the polymerisate.

[0113] Again, the operation was performed after switching the strainer. The next day, the cavitation of the pump occurred again.

[0114] Thereafter, the operation with switching the strainer after the occurrence of cavitation resulted in a shorter interval of the occurrence of cavitation and finally the operation became impossible.

[0115] The operation of the distillation column was stopped and the inside of the distillation column was checked. As a result, a large amount of polymerisate was confirmed.

INDUSTRIAL APPLICABILITY

[0116] Use of the apparatus for extracting a liquid according to the present invention enables extraction of distillation residue at the column bottom and a condensate from a reflux tank of the column top from, for example, a vacuum distillation apparatus for a polymerizable substance without any trouble to the operation of the vacuum distillation apparatus.

[0117] Further, according to the present invention, by measuring a differential pressure between the pressure on the upstream side of the strainer and the pressure of the downstream side of the strainer installed in the top of the liquid extraction pipe connecting the treating column used for the treating apparatus for handling an easily polymerizable substance and the pump, and determining the times of switching strainers, cavitation of the pump during the operation can be avoided, thereby preventing variation in the operation of the pump or the appliance connected before and after the pump and providing a process for efficiently producing an easily polymerizable substance.

[0118] Further, according to the apparatus and the method for handling an easily polymerizable substance of the present invention, in the handling of the easily polymerizable substance, the troubles of operation caused by the air contained in the backup strainer can be avoided; two or more strainers parallelly installed in the liquid extracting pipe for transferring the outlet liquid of the treating column from the treating column for handling an easily polymerizable substance to the pump for transferring the outlet liquid of the treating column can be switched at proper times; or the aforementioned troubles in the operation can be avoided and the aforementioned strainers can be switched at proper times. 

1. An apparatus for handling an easily polymerizable substance, comprising: a decompressor for handling the easily polymerizable substance under reduced pressure; a liquid extraction pipe having an upstream end that opens in the decompressor; a pump provided in the middle of the liquid extraction pipe; a plurality of strainers connected in parallel one another to the liquid extraction pipe on the upstream side of the pump; and valves provided on the upstream side and downstream side of each strainer, wherein the apparatus comprises at least one of constructions (A) and (B) below: (A) an exhaust means for exhausting air in a part sandwiched between the valves provided on the upstream side and downstream side of each strainer, and a branch pipe that connects the part sandwiched between the valves provided on the upstream side and downstream side of each strainer and the liquid extraction pipe on the downstream side of the pump; and (B) a means for measuring a differential pressure between a pressure in the liquid extraction pipe on the upstream side of each strainer and a pressure in the liquid extraction pipe on the downstream side of each strainer.
 2. The apparatus according to claim 1, wherein the exhaust means is configured such that when a liquid is introduced from the branch pipe to the part sandwiched between the valves provided on the upstream side and downstream side of each strainer, the liquid expels the air in the part.
 3. The apparatus according to claim 1, wherein the decompressor for handling the easily polymerizable substance under reduced pressure is an apparatus for distilling the easily polymerizable polymer selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof under reduced pressure.
 4. A method for handling an easily polymerizable substance using the apparatus according to claim 1, comprising: using the plurality of strainers one by one; determining the time for switching the strainers to measure a difference between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used; and switching the strainer used to another strainer out of the plurality of strainers.
 5. The method according to claim 4, further comprising: discharging air in a part sandwiched between valves provided on the upstream side and downstream side of the another strainer; feeding a discharge liquid of the pump from the branch pipe to the part; and switching the strainer used to another strainer after replacing the air in the part with the discharge liquid.
 6. An apparatus for extracting a liquid from an apparatus under reduced pressure, comprising: a liquid extraction pipe for transferring a liquid under reduced pressure contained in an apparatus under reduced pressure, having an upstream end that opens in the apparatus under reduced pressure, and having a pump in the middle of the pipe; a plurality of strainers connected in parallel one another to the liquid extraction pipe on the upstream side of the pump; valves provided on the upstream side and downstream side of each strainer; an exhaust means for exhausting air in a part sandwiched between the valves; and a branch pipe that connects a part sandwiched between the valves provided on the upstream side and downstream side of each strainer and the liquid extraction pipe on the downstream of the pump.
 7. The apparatus according to claim 6, wherein the exhaust means is configured such that when a liquid is introduced from the branch pipe to the part sandwiched between the valves provided on the upstream side and downstream side of each strainer, the liquid expels the air in the part.
 8. The apparatus according to claim 6, wherein the apparatus under reduced pressure is a vacuum distillation apparatus for an easily polymerizable substance selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof.
 9. A process for producing an easily polymerizable substance using a treating apparatus having one or two or more treating columns for treating the easily polymerizable substance, a pump provided in each treating column for transferring an outlet liquid of the treating column, and two or more strainers installed parallelly in a liquid extraction pipe connecting the treating column and the pump, wherein the process comprises: using the two or more strainers one by one in the each treating column; determining the time of switching the strainers to measure a difference between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used; and switching the strainer used to another strainer out of the two or more strainers.
 10. The process according to claim 9, wherein the differential pressure of the strainer is continuously measured.
 11. The process according to claim 9, wherein extract nozzles for connecting a detection end for measuring pressure are installed to the upper part of the liquid extraction pipe of the strainer used.
 12. The process according to claim 9, wherein the easily polymerizable substance is selected from the group consisting of acrylic acid, methacrylic acid, and esters thereof.
 13. The process according to claim 9, wherein the treating apparatus comprises the treating column selected from any one of a distillation column, an evaporation column, a reflux tank, an extraction column, an absorption column, a high boiling point decomposition reactor, an esterification reactor, a heat exchanger or combinations thereof.
 14. A method for handling an easily polymerizable substance using the apparatus according to claim 2, comprising: using the plurality of strainers one by one; determining the time for switching the strainers to measure a difference between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used; and switching the strainer used to another strainer out of the plurality of strainers.
 15. A method for handling an easily polymerizable substance using the apparatus according to claim 3, comprising: using the plurality of strainers one by one; determining the time for switching the strainers to measure a difference between a pressure in the liquid extraction pipe on the upstream side of the strainer used and a pressure in the liquid extraction pipe on the downstream side of the strainer used; and switching the strainer used to another strainer out of the plurality of strainers. 